PURPOSE: To execute easily a high-stability and high-precision experiment, by caulculating the movement speed of a reflective body on a basis of the speed of revolution of a driving means to perform the calibration and the operation confirmation.

CONSTITUTION: A speed detector 14 is so attached that a beam is irradiated at a point P, which is a length (r) apart from a center point O of a supporting body 12, on the supporting body 12, and the speed at the point P is detected. A shaft 6 to which a reflective body 3 is attached is rotated and driven by a motor 7 whose speed of revolution is controlled by a speed controlling part 9. Simultaneously, its number of revolution is detected by a number of revolution detector 8 and is converted to the number of revolution by a number of revolution converter 10. The supporting body 12 is fixed to the shaft 6 by a bearing 11. A detector of speedomer to be measured is attached to the position of the speedometer 14, and the indication on the number of revolution converter 10 is read to calculate a number of revolution (n) simultaneously with calculation of the length (r) between points O and P, and thus, a reference speed is calculated.